Host cell processes to accomplish mechanical and non-circulative virus transmission

HCPro-mediated transmission by aphids of purified virions does not require its silencing suppression function and correlates with its ability to coat cell microtubules in loss-of-function mutant studies

Native and amino acid (aa) substitution mutants of HCPro from potato virus Y (PVY) were transiently expressed in Nicotiana benthamiana leaves. Properties of those HCPro variants with regard to silencing suppression activities, mediation of viral transmission by aphids, and subcellular localization dynamics, were determined. One mutant failed to suppress silencing in agropatch assays, but could efficiently mediate the transmission by aphids of purified virions. This mutant also retained the ability to translocate to microtubules (MTs) in stressed cells. By contrast, another single aa substitution mutant displayed native-like silencing suppression activity in agropatch assays, but could not mediate transmission of PVY virions by aphids, and could not relocate to MTs. Our data show that silencing suppression by HCPro is not required in the aphid-mediated transmission of purified virions. In addition, since the same single aa alteration compromised both, viral transmission and coating of MTs, those two properties could be functionally related.

Localizing viruses in their insect vectors

The mechanisms and impacts of the transmission of plant viruses by insect vectors have been studied for more than a century. The virus route within the insect vector is amply documented in many cases, but the identity, the biochemical properties, and the structure of the actual molecules (or molecule domains) ensuring compatibility between them remain obscure. Increased efforts are required both to identify receptors of plant viruses at various sites in the vector body and to design competing compounds capable of hindering transmission. Recent trends in the field are opening questions on the diversity and sophistication of viral adaptations that optimize transmission, from the manipulation of plants and vectors ultimately increasing the chances of acquisition and inoculation, to specific "sensing" of the vector by the virus while still in the host plant and the subsequent transition to a transmission-enhanced state.

Plant pararetroviruses: interactions of cauliflower mosaic virus with plants and insects

Virion associated protein (VAP) binds to the icosahedral capsid of cauliflower mosaic virus (CaMV) - a plant pararetrovirus. The interactive coiled-coil domains of this protein can interact with the coiled-coils of either the movement protein or the aphid transmission factor, thereby mediating both cell-to-cell movement and aphid transmission. The host counters CaMV infection with two lines of defense: innate immunity and silencing. The viral protein 'transactivator/viroplasmin' (TAV) is recognized as an effector and either initiates the innate immunity reaction in a non-permissive host or interferes with it in a permissive host. As a silencing suppressor, TAV interferes with dicing of dsRNAs.

Microtubules in viral replication and transport

Viruses use and subvert host cell mechanisms to support their replication and spread between cells, tissues and organisms. Microtubules and associated motor proteins play important roles in these processes in animal systems, and may also play a role in plants. Although transport processes in plants are mostly actin based, studies, in particular with Tobacco mosaic virus (TMV) and its movement protein (MP), indicate direct or indirect roles of microtubules in the cell-to-cell spread of infection. Detailed observations suggest that microtubules participate in the cortical anchorage of viral replication complexes, in guiding their trafficking along the endoplasmic reticulum (ER)/actin network, and also in developing the complexes into virus factories. Microtubules also play a role in the plant-to-plant transmission of Cauliflower mosaic virus (CaMV) by assisting in the development of specific virus-induced inclusions that facilitate viral uptake by aphids. The involvement of microtubules in the formation of virus factories and of other virus-induced inclusions suggests the existence of aggresomal pathways by which plant cells recruit membranes and proteins into localized macromolecular assemblies. Although studies related to the involvement of microtubules in the interaction of viruses with plants focus on specific virus models, a number of observations with other virus species suggest that microtubules may have a widespread role in viral pathogenesis.

Early interactions during the encounter of plants, aphids and arboviruses

Aphids infest many plants and cause damage by depriving them of nutrients and by transmitting many viral diseases. Aphid infestation and arbovirus transmission are controlled by establishment (or not) of a compatible reaction between the insects and the plants. This reaction is the result of defense reactions of the plant and counter-defense reactions of the parasite. Contrarily to plant-bacteria, plant-fungi and plant-herbivorous insects pathosystems, the plant-aphid pathosystem is understudied, although recent advances have begun to uncover some of its details. Especially the very early steps in plant-aphid interactions are hardly known. We here resume the present knowledge of these interactions. We discuss further how an aphid-transmitted plant virus that is transmitted during the first moments of the plant-aphid encounter, might help to study the very early plant aphid interactions.

A virus responds instantly to the presence of the vector on the host and forms transmission morphs

Many plant and animal viruses are spread by insect vectors. Cauliflower mosaic virus (CaMV) is aphid-transmitted, with the virus being taken up from specialized transmission bodies (TB) formed within infected plant cells. However, the precise events during TB-mediated virus acquisition by aphids are unknown. Here, we show that TBs react instantly to the presence of the vector by ultra-rapid and reversible redistribution of their key components onto microtubules throughout the cell. Enhancing or inhibiting this TB reaction pharmacologically or by using a mutant virus enhanced or inhibited transmission, respectively, confirming its requirement for efficient virus-acquisition. Our results suggest that CaMV can perceive aphid vectors, either directly or indirectly by sharing the host perception. This novel concept in virology, where viruses respond directly or via the host to the outside world, opens new research horizons, that is, investigating the impact of ‘perceptive behaviors’ on other steps of the infection cycle.

DOI:http://dx.doi.org/10.7554/eLife.00183.001

Viruses are infectious agents that can replicate only inside a living host cell. When a virus infects an animal or plant, it introduces its own genetic material and tricks the host cells into producing viral proteins that can be used to assemble new viruses. An essential step in the life cycle of any virus is transmission to a new host: understanding this process can be crucial in the fight against viral epidemics.

Many viruses use living organisms, or vectors, to move between hosts. In the case of plant viruses such as cauliflower mosaic virus, the vectors are often aphids. When an aphid sucks sap out of a leaf, virus particles already present in the leaf become attached to its mouth, and these viruses can be transferred to the next plant that the insect feeds on. However, in order for cauliflower mosaic virus particles to become attached to the aphid, structures called transmission bodies must form beforehand in the infected plant cells. These structures are known to contain helper proteins that bind the viruses to the mouth of the aphid, but the precise role of the transmission body has remained obscure.

Now Martinière et al. show that the transmission body is in fact a dynamic structure that reacts to the presence of aphids and, in so doing, boosts the efficiency of viral transmission. In particular, they show that the action of an aphid feeding on an infected leaf triggers a rapid and massive influx of a protein called tubulin into the transmission body. The transmission body then bursts open, dispersing helper protein-virus particle complexes throughout the cell, where they become more accessible to aphids. This series of events increases viral transmission rates twofold to threefold.

The results show that a virus can detect insect vectors, likely by using the sensory system of its host, and trigger a response that boosts viral uptake and thus transmission. This is a novel concept in virology. It will be important to discover whether similar mechanisms are used by other viruses, including those that infect animals and humans.

DOI:http://dx.doi.org/10.7554/eLife.00183.002